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At present there is a "constellation" of 66 active Iridium communications satellites and nine spares, all moving along in nearly circular orbits at an altitude of about 780 km above Earth's surface. Each Iridium satellite has three highly-reflective, mirror-like antennas, called "Main Mission Antennas" (MMAs).
Each MMA is tilted down 40 degrees from the long axis of the spacecraft, and the long axis always points down toward the center of the Earth. Should you be looking up in the direction of one of these MMAs and see the reflection of the Sun through it, you will observe, for a few fleeting seconds, a bright flare of sunlight that can exceed visual magnitude -8. That's about 20 times brighter than the planet Venus at its brightest.
That bright flare of sunlight, moving swiftly in front of a relatively fixed star background, is what is called an "Iridium flare." If you have not seen one, let me tell you, a bright Iridium flare is one of the most remarkable and exciting visual phenomena than any skywatcher can see.
How to Make Iridium Flare Predictions
You can now make Iridium flare predictions on your own computer using a new program from Software Bisque called TheSkyX (see http://www.bisque.com).
TheSkyX's Iridium flare prediction capability is based upon the rev-oriented computing engine in Space Birds, a satellite tracking and visibility prediction program that I wrote in 1987. Space Birds was distributed during 1988-1991 by Sky Publishing Corp., publisher of the monthly Sky & Telescope magazine.
The Iridium flare prediction problem is one of those problems for which one must resort to computer code. Indeed, to predict all of the flares for a single observer for one week, one must consider in great detail all of the visible passes of the 75 active or spare Iridium satellites, for approximately 100 orbital revolutions. I used Borland's C++ Builder 5 to develop the code for this problem.
How, then, did Mathcad play a role in my efforts? There were two areas for which Mathcad was indispensable, as described below.
Mathcad's Genfit Captures the Parameters of a Nonlinear Curve
To calculate the logarithmic visual magnitude (an estimate of how bright the flare is) at each instant of a flare, I relied on the prior work of Randy John, who wrote the SKYSAT program for predicting Iridium flares. I needed to capture the parameters of Randy's magnitude versus mirror angle curve without resorting to Excel, which I did not have at hand. Mathcad 13's genfit proved perfect for the task. The worksheet by which I extracted the logarithmic curve parameters is attached.
This worksheet constructs a mapping of magnitude vs. mirror angle for an Iridium flare. Source of the data is the graph http://home.comcast.net/~skysat/IRID_PLOT2.htm by Randy John, author of SKYSAT [1]. | 
